Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.
For example:
Given a binary tree{1,2,3,4,5},
1
/ \
2 3
/ \
4 5
return the root of the binary tree[4,5,2,#,#,3,1].
4
/ \
5 2
/ \
3 1
刷晕了。。。。 1， 一个节点的左孩子等于他的兄弟节点， 右孩子等于父节点。
第一种迭代方法， 比较好容易理解还节省栈空间
